What will be the output of the below java code using volatile keyword?

Problem statement: Given code is

1. public class Test extends Thread {
2. private volatile int i = 0;
3. public int getData() {
4. return i + 1;
5. }
6. public void run() {}
7. public static void main(String[] args) {
8. Test t1 = new Test();
9. t1.start();
10. System.out.println("First thread: " + t1.getData());
11. Test t2 = new Test();
12. t2.start();
13. System.out.println("Second thread: " + t2.getData());
14. Test t3 = new Test();
15. t3.start();
16. System.out.println("Third thread: " + t3.getData());
17. }
18. }

Output:
First thread: 1
Second thread: 1
Third thread: 1

Example for compareTo() method?

Reference

How do you sort collection of StringBuffer object in java?

Problem statement: Given the collection of StringBuffer objects.
StringBuffer sb1 = new StringBuffer("Z");
StringBuffer sb2 = new StringBuffer("A");
StringBuffer sb3 = new StringBuffer("P");

You have to sort it either way, meaning natural order or custom order.

Answer:
// ASCENDING ORDER:

1. import java.util.Comparator;
2. import java.util.TreeSet;
3. public class Test {
4. public static void main(String[] args) {
5. SortStringBufferObject s = new SortStringBufferObject();
6. TreeSet<StringBuffer> t = new TreeSet<>(s);
7. t.add(new StringBuffer("Z"));
8. t.add(new StringBuffer("A"));
9. t.add(new StringBuffer("P"));
10. System.out.println(t);
11. }
12. }
13. class SortStringBufferObject implements Comparator<StringBuffer> {
14. public int compare(StringBuffer sb1, StringBuffer sb2) {
15. return sb1.toString().compareTo(sb2.toString());
16. }
17. }

Output:

[A, P, Z] 

// DESCENDING ORDER:

1. import java.util.Comparator;
2. import java.util.TreeSet;
3. public class Test {
4. public static void main(String[] args) {
5. SortStringBufferObject s = new SortStringBufferObject();
6. TreeSet<StringBuffer> t = new TreeSet<>(s);
7. t.add(new StringBuffer("Z"));
8. t.add(new StringBuffer("A"));
9. t.add(new StringBuffer("P"));
10. System.out.println(t);
11. }
12. }
13. class SortStringBufferObject implements Comparator<StringBuffer> {
14. public int compare(StringBuffer sb1, StringBuffer sb2) {
15. return sb2.toString().compareTo(sb1.toString());
16. }
17. }

Output:

[Z, P, A]
Reference

What is the internal working of Set / HashSet in Java ?

Problem statement: Give an explanation for internal working of Set / HashSet in java.
Answer: Reference

What will be the output when you add custom elements in HashSet?

Problem statement: Given code is

1. import java.util.HashSet;
2. class A {}
3. class B {}
4. class C {}
5. public class Test {
6. public static void main(String[] args) {
7. HashSet set = new HashSet();
8. set.add(new A());
9. set.add(new B());
10. set.add(new C());
11. set.add(new A());
12. System.out.println("Elements: \n" + set);
13.   System.out.println("Size: " + set.size());
14. }
15. }

Answer:
Elements: 
[com.practice.core.C@7852e922, com.practice.core.A@15db9742, com.practice.core.B@6d06d69c, com.practice.core.A@4e25154f]
Size: 4

What will be the output when you add String object in HashSet?

Problem statement: Given code is 

1. public class Test {
2. public static void main(String[] args) {
3. HashSet<String> s = new HashSet<>();
4. s.add("A");
5. s.add("B");
6. s.add("C");
7. s.add("A");
8.   System.out.println("Elements: \n" + s);
9.   System.out.println("Size: " + s.size());
10.   System.out.println("Objects: " + s);
11. }
12. }

Answer:
Elements: 
[A, B, C]
Size: 3
Objects: [A, B, C]

What will be the output when we add elements in collection of ArrayList in java?

Problem statement: In given java code, can you please let me know, what will be the output.

1. import java.util.ArrayList;
2. class A {}
3. class B {}
4. class C {}
5. public class Test {
6. public static void main(String[] args) {
7. ArrayList list = new ArrayList();
8. list.add(new A());
9. list.add(new B());
10. list.add(new C());
11.   list.add(new A());
12. System.out.println(list);
13.   System.out.println("Size: " + list.size());
14. }
15. }

Answer:
[com.practice.core.A@15db9742, com.practice.core.B@6d06d69c, com.practice.core.C@7852e922, com.practice.core.A@4e25154f]
Size: 4

Note: fully_qualified_class_name@hashcode_value